Constraints on mineralization, fluid-rock interaction, and masstransfer during faulting at 2-3 km depth from the SAFOD drill hole

摘要

Mineralogical and geochemical changes in mudrock cuttings from two segments ofthe San Andreas Fault Observatory at Depth (SAFOD) drill hole (3066-3169 and3292-3368 m measured depth) are analyzed in this study. Bulk rock samples andhand-picked fault-related grains characterized by polished surfaces and slickensides wereinvestigated by X-ray diffraction, electron microscopy, and geochemical analysis. Theelemental changes in fault-related grains along the sampled San Andreas Fault areattributed to dissolution of detrital grains (particularly feldspar and quartz) and localprecipitation of illite-smectite and/or chlorite-smectite mixed layers in fractures and veins.Assuming Zr02 and TiO2 to be immobile elements, systematic differences in elementconcentrations show that most of the elements are depleted in the fault-relatedgrains compared to the wall rock lithology. Calculated mass loss between the bulk rockand picked fault rock ranges from 17 to 58% with a greater mass transport in theshallow trace of the sampled fault that marks the upper limit the fault core. The relativelylarge amount of element transport at temperatures of —110-114°C recordedthroughout the core requires extensive fluid circulation during faulting. Whereasdissolution/precipitation may be partly induced by the disequilibrium between fluids androcks during diagenetic processes, stress-induced dissolution at grain contacts is proposedas the main mechanism for extensive mineral transformation in the fault rocks andlocalization of neomineralization along grain interface slip surfaces.